Coastal bays are critical transitional zones characterized by intensive land-sea interactions, and nutrient pollution has become a major constraint on the ecological and environmental security of coastal regions. Existing studies have largely focused on the cost-effectiveness of land-based nutrient abatement measures, such as agricultural management practices, whereas systematic assessments of nutrient abatement measures under an integrated land–sea management framework remain limited. This knowledge gap has hindered the development of effective strategies for controlling nitrogen and phosphorus pollution in coastal bays.
To address this issue, this study first identifies the major sources of nitrogen and phosphorus pollution affecting coastal bays. A large language model (LLM) is then used to assist in identifying the principal abatement measures associated with each emission source. Multi-source data from scientific literature, engineering and technical reports, and field investigations are subsequently integrated to construct a cost-effectiveness database of nutrient abatement measures. Taking Sansha Bay, a typical semi-enclosed bay in Fujian Province, China, as a case study, the research further conducts a quantitative assessment of the cost-effectiveness of nutrient pollution abatement measures under an integrated land–sea management framework. It identifies the applicable conditions and abatement-scale constraints of different measures and establishes a priority ranking of measures under alternative abatement-target scenarios. The findings are expected to provide quantitative decision support and methodological guidance for the effective management of nitrogen and phosphorus pollution in coastal bays.